The present invention relates generally to an apparatus for communicating pressure from an external fluid to a pressure sensitive tool and, more particularly, but not by way of limitation, to a filter cartridge for interfacing pressure exerted by a bore hole fluid with a pressure sensitive tool.
In drilling, completing, testing and producing oil or gas wells, there are various objects which are used within the bore hole of the well and to which an external pressure needs to be communicated. Certain types of these objects preferably should not have an external fluid, which is in the bore hole and through which the pressure is exerted, communicated to the object. An example of such a certain type of object is a pressure transducer used for logging or testing to record downhole pressure within the bore hole of an oil or gas well. One specific type of pressure transducer employs a crystal pressure sensing element which is sensitive to the pressure applied to it.
To convey the bore hole pressure to such a pressure sensing element without introducing hysteresis, a long narrow passageway defined by a small diameter coiled tubing, generally known as a "buffer tube," is filled with a clean fluid and exposed at one end to the bore hole fluid. This tubing is connected at its other end to the housing in which the crystal pressure sensing element is held so that the clean fluid in the tubing communicates with a volume of clean fluid surrounding the pressure sensing element. Thus, the coiled tubing defines a single passageway through which the pressure from the bore hole fluid is to be communicated to the pressure sensing element.
This single passageway of the coiled tubing is necessarily of small internal diameter (e.g., 0.06 inch) so that the tubing holds the clean fluid under a condition of pressure equilibrium, but it is large enough (if not blocked) to communicate the pressure from the bore hole fluid through the clean fluid.
When the logging or testing tool is subjected to temperature changes as it is lowered or raised to different depths of the bore hole, the clean fluid is also subjected to such temperature changes. When the change heats the clean fluid, it expands. This expansion forces a quantity of the clean fluid out of the small passageway at the end of the coiled tubing communicating with the bore hole fluid. When the change cools the clean fluid, the fluid contracts, thereby sucking external fluid, including some of the bore hole fluid (which may be drilling mud or other particulate containing mixture) back into the coiled tubing. If enough of the bore hole fluid particles are sucked into the small passageway of the coiled tubing, this passageway becomes blocked, thereby preventing further communication of the bore hole pressure to the pressure sensing element.
To alleviate this problem whereby the single passageway of the coiled tubing becomes blocked, there is the need for an interface which effectively communicates the pressure through a plurality of passageways while isolating the external fluid from the pressure sensing element so that it is less likely all of the plurality of passageways will be blocked under conditions during which a portion of the external fluid may be sucked into one or more of the passageways.